Fixed link to replying to other replies in the diff viewer

Review Request #822 — Created Aug. 15, 2017 and updated

Review Board
On the diff page the list of comments doesn't differenciate between top-level
comments and replies to those comments. Because reviewboard doesn't support
replying to a reply of a comment, the "Reply" link on those reply comments
would not work properly. This fix solves that problem by changing those reply
links to reply to their parent comment.

Testing Done:
Manual Test:
1. Create a comment on a line on the diff viewer page for a review request.
2. Reply to that comment.
3. Go back to the diff viewer page and click the reply link of that reply
4. Expected result is that it should go to the review page, scroll to, and open
   the reply editor for the parent comment of that reply.

- Jasmine js test suite passed, with 2 new tests added.
- Reviewboard unit test suite passed.

Bugs closed: 4049

Reviewed at